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Equipment Discussions >> Mounts

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jimsmith
member


Reged: 10/11/07

Loc: Herefordshire, UK
Reducing mount vibrations. new
      #5871083 - 05/19/13 10:30 AM Attachment (69 downloads)

I have a WO EZTouch mount on a fixed steel pier. There are two telescopes mounted permanently on it...a 250mm mak and 120mm refractor. The views through the mak are quite stable even at higher magnifications, but the refractor wobble can be quite excessive at higher magnifications. It takes about 2-3 seconds for it to settle down after focussing.

The mak is mounted on the "native" vixen side of the mount. The refractor is mounted using an accessory vixen dovetail attachment screwed/bolted to the other side. I am using the SkyWatcher provided vixen bar and tube rings. The attached photo shows the relevant bit of the set-up.

Would I be likely to get better stability if I mounted the refractor using a Losmandy type plate or perhaps tube rings spaced further apart...or closer together...or...?

I wonder if anyone has any recommendations before I start investing in new kit that might not make any significant difference.

Thanks, Jim


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Whichwayisnorth
Pooh-Bah
*****

Reged: 07/04/11

Loc: Southern California
Re: Reducing mount vibrations. new [Re: jimsmith]
      #5871633 - 05/19/13 02:39 PM

I wish I could help. Looks like you are already doing what I would have done.

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EFT
Vendor - Deep Space Products
*****

Reged: 05/07/07

Loc: Phoenix, AZ
Re: Reducing mount vibrations. new [Re: jimsmith]
      #5873455 - 05/20/13 10:45 AM

This is really a mount issue and not a scope and dovetail issue. The majority of the problem that you are having comes from the fact that the refractor has a much longer moment arm than the MAK. On a light weight mount like this, that equals stability problems. A refractor of weight equal to that of the MAK, but much greater length will be less stable than the MAK in most cases. While you might see some flexure in the refractor mounting, it is not likely to be anywhere near the amount of flexure or axis instability that you are seeing from the mount. Unfortunately, for this load, you are looking a needing a more hefty and stable mount.

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gdd
Pooh-Bah


Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: Reducing mount vibrations. new [Re: jimsmith]
      #5873758 - 05/20/13 01:22 PM

Hi Jim,

Maybe motorizing the focuser will allow this mount to work with the 120mm refractor.

Gale


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dhaval
professor emeritus
*****

Reged: 07/21/08

Loc: Round Rock, TX
Re: Reducing mount vibrations. new [Re: gdd]
      #5873765 - 05/20/13 01:25 PM

The best way to reduce mount vibrations is to try and not have them in the first place! I would say you are probably under mounted. You really need a beefier mount!

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jimsmith
member


Reged: 10/11/07

Loc: Herefordshire, UK
Re: Reducing mount vibrations. new [Re: dhaval]
      #5873785 - 05/20/13 01:35 PM

Yes, I'm afraid that EFT and dhaval are probably right. I've got too big a load on the mount. Are there any similar but beefier mounts out there?

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jimsmith
member


Reged: 10/11/07

Loc: Herefordshire, UK
Re: Reducing mount vibrations. new [Re: jimsmith]
      #5874034 - 05/20/13 03:47 PM

Motorising the focuser is an interesting alternative, but I'm not really enthusiastic about anything that makes my set-up more complicated.

I carried out a few experiments...
Removing the Mak seemed to make no difference.
Moving the refractor to the slightly shorter mount arm made the vibrations worse. (I wasn't expecting that.)
Sliding the refractor up and down in its rings made no difference to the vibrations.

It looks like I may have to live with the problem, or spend my way out of it. The prices on the aokswiss.ch site for the bigger mounts look a bit scary.


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Mark9473
Postmaster
*****

Reged: 07/21/05

Loc: 51°N 4°E
Re: Reducing mount vibrations. new [Re: jimsmith]
      #5874133 - 05/20/13 04:16 PM

Is the steel pier filled with sand?

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jimsmith
member


Reged: 10/11/07

Loc: Herefordshire, UK
Re: Reducing mount vibrations. new [Re: Mark9473]
      #5874222 - 05/20/13 04:50 PM

No, the steel pier is not filled with sand, although I did have it made with a hole in the top in case filling it turned out to be necessary. The pier does ring if you hit it. but that's at a much higher frequency than the visible vibrations. (About 6 or 7 wobbles per second.) The Mak seems fine so I assumed the pier was OK. Am I perhaps mistaken?

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StarStuff1
Post Laureate
*****

Reged: 04/01/07

Loc: South of the Mason-Dixon Line
Re: Reducing mount vibrations. new [Re: jimsmith]
      #5874241 - 05/20/13 05:02 PM

It certainly would not hurt to fill the pier with sand. Years ago I had a ROR observatory with a 5-in apo with an EQ mount on top of a 4-in square hollow steel pier. Sand definitely helped dampen vibrations in that case.

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Dwight J
scholastic sledgehammer
*****

Reged: 05/14/09

Loc: Lethbridge, Alberta, Canada
Re: Reducing mount vibrations. [Re: StarStuff1]
      #5874281 - 05/20/13 05:26 PM

One problem with using sand is moisture retention and this can cause the pier to rust if it is steel. No doubt it would add mass. An alternative would be low expansion foam which will absorb some of the vibration and pose no rust risk. You could add turnbuckles that go from the top of the pier to the end of the pier legs/feet and once tightened, can reduce movement. Separating the tube rings further apart could also help.

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jimsmith
member


Reged: 10/11/07

Loc: Herefordshire, UK
Re: Reducing mount vibrations. new [Re: Dwight J]
      #5876016 - 05/21/13 02:12 PM

Thanks for more good ideas.
The steel pier is concreted into a deep hole, so it has no legs or feet.
As a long shot, I have just ordered some 3mm Sorbothane sheet which I will cut into strips and use between the scope and the tube rings. It might make some difference I suppose...


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jimsmith
member


Reged: 10/11/07

Loc: Herefordshire, UK
Re: Reducing mount vibrations. new [Re: jimsmith]
      #5884169 - 05/25/13 03:11 PM

The sorbothane sheet was too thick to use in the way I intended. So no progress there.

However, I did change the position of both scopes slightly (I moved one forward and the other back) and that seems to have reduced the vibrations somewhat.

I think I can live with the slight wobbliness now.


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Per Frejvall
sage


Reged: 09/28/12

Loc: Saltsjöbaden, Sweden
Re: Reducing mount vibrations. new [Re: jimsmith]
      #5885118 - 05/26/13 04:44 AM Attachment (21 downloads)

I notice a quite long Vixen bar... Apart from pier oscillations, that will be the source of problems. I haven't used a Vixen bar since I discovered the Losmandy bars - no comparison!

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jimsmith
member


Reged: 10/11/07

Loc: Herefordshire, UK
Re: Reducing mount vibrations. new [Re: Per Frejvall]
      #5885177 - 05/26/13 07:00 AM

Thanks Per. Your thoughts match my original thinking on the problem. I will find out how much the Losmany alternative would cost me.

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jimsmith
member


Reged: 10/11/07

Loc: Herefordshire, UK
Re: Reducing mount vibrations. new [Re: jimsmith]
      #6460072 - 04/11/14 10:39 AM

Nearly a year on and I'm finally going to get around to fixing this problem. Mars would look even better without the wobble. It's pretty bad at x180. This week is going to be the week I fill the steel pier with sand. Best to try the cheap option first, I think.

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jimsmith
member


Reged: 10/11/07

Loc: Herefordshire, UK
Re: Reducing mount vibrations. new [Re: jimsmith]
      #6469816 - 04/16/14 02:02 PM

So, I filled the pier with sand. Now when I tap the pier it no longer makes a metallic ringing sound. So high frequency vibrations are now being suppressed. Unfortunately, it has made no difference to the visual wobble. I wonder what to try next.

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Xeroid
super member
*****

Reged: 09/21/13

Loc: USA
Re: Reducing mount vibrations. new [Re: jimsmith]
      #6470082 - 04/16/14 04:45 PM

Ok, you asked what's next, here goes a crazy suggestion:

1. Replace the metal ring dovetail bar with a LONGER HARDWOOD (White Oak) bar shaped to mimic the dovetail angles (12 degrees?)

Why?
Increasing the space between the telescope rings should help to reduce flex/wobble.
Longer dovetail made of wood will dampen vibration while reducing weight.


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gdd
Pooh-Bah


Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: Reducing mount vibrations. new [Re: Xeroid]
      #6470295 - 04/16/14 07:03 PM

No matter how long the dovetail the length of the attachment area on the mount remains the same. Try suspending a chain from the long end of the dovetail or from either end of the OTA to absorb vibrations. Let the chain drag on the ground. Be sure to protect the OTA from the chain.

Here is a CN thread containing the chain idea and a few others to dampen telescope vibrations.

Gale

Edited by gdd (04/16/14 10:45 PM)


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Dan Finnerty
sage


Reged: 09/11/11

Loc: Pasadena, CA
Re: Reducing mount vibrations. new [Re: jimsmith]
      #6470398 - 04/16/14 08:12 PM

Quote:

So, I filled the pier with sand. Now when I tap the pier it no longer makes a metallic ringing sound. So high frequency vibrations are now being suppressed. Unfortunately, it has made no difference to the visual wobble. I wonder what to try next.




I have never tried this myself but I have heard reports of this working well. You can make a diagonal brace that extends from the objective end of the telescope tube to the counterweight shaft. This can very effectively damp out vibrations due to a long telescope tube with a lot of weight (the objective lens) at the end of the tube.

There is a name for this strut, but I cannot recall it. I am sure others will be able to remember it and then you can do a google search and get more information than you ever wanted!


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